Method of treating stainless steel to improve its corrosion resistance



United States PatentfO METHOD OF TREATING STAINLESS STEEL TO IMPROVE ITS CORROSION RESISTANCE Ellen L. Karchner, Grand Rapids, Mich., assignor to General Motors Corporation, Detroit, Micl1., a corporation of Delaware No Drawing. Application November 25, 1955 Serial No. 549,198

8 Claims. (Cl. 148-616) This invention relates to the treatment of stainless steel and is more particularly concerned with treating stainless steel in order to improve the corrosion resistance thereof.

The primary object of this invention is to provide an improved method and solution for treating stainless steels and more particularly to provide an improved method and solution for treating stainless steels of the types commonly referred to as free machining grades to improve the corrosion resistance thereof. The free machining grades contain sulphur or selenium to improve the ma chining characteristics. Other objects and advantages of the invention will become more apparent as the description proceeds.

The stainless steels are steels containing relatively large amounts of chromium or relatively large amounts of both nickel and chromium. In general stainless steels contain at least 6% nickel together with larger amounts of chromium or at least about 11.0% chromium for a straight chromium type stainless steel. type stainless steels also may contain a small percentage of nickel. A typical example of a stainless steel containing both nickel and chromium is one containing about 8 to 10% nickel and 17 to 19% chromium. A typical example of a stainless steel of the straight chromium type is one containing 11.5 to 27.0% chromium.

In accordance with the present invention there is provided a new and improved solution and method for treating stainless steel surfaces. The solution employed is an aqueous solution composed of about 3 to 10% chromic acid by weight and about 2 to phosphoric acid by weight (commercial orthophosphoric acid containing 85% acid). The optimum solution is an aqueous solution containing about 3 to 5% chromic acid by weight and 2% phosphoric acid by weight. The stainless steel to be treated is immersed in the solution for a short time.

A preferred treatment consists of immersing the stainless steel part in the aqueous solution of chromic acid and phosphoric acid at a temperature of 180 F. for twenty minutes. optimum one considerable variation in temperature is permissible. For example, very satisfactory results may be obtained with bath temperatures ranging from 170 F. to 190 F.

The time of immersion also may vary considerably and times both greater and lesser than twenty minutes may be employed. In general improved corrosion resistance is obtained with treatment times as low as minutes. Times as high as one hour or more may be employed but since a time of 20 minutes is adequate it is undesirable in commercial applications to increase the time much above 20 minutes. This is particularly true since little if any further improvement is obtained from longer periods of immersion than 20 minutes.

While the invention is applicable to all types or grades of stainless steel it is particularly advantageous in connection with the types of stainless steel which are commonly referred to as free machining grades. These The chromium While a temperature of 180 F. is the I 'ice Percent Percent 0.25 Max- 2.00 Max. 0.04 Max 0.03 Max--- 1.50 Max" 24.0-26.0"... 19.0-22.0..."

Percent 0.15 Man--- M0 or Zr 0.60

Max. balance.

Others Iron , 1 ligsloth 303 and 416 S may be replaced by Se and the usual range is Tests indicate that the corrosion resistance of stainless steel is improved greatly by means of the present treatment. For example, tests made on 416 stainless steel parts indicate that ground surfaces when treated in accordance with the method of this invention show no rusting after 30 hours in a salt spray whereas similar parts which have been processed by a conventionally known method of passivating the parts with nitric acid show rusting after 20 minutes in the same salt spray test.

Numerous changes and modifications of the embodiments of the invention described herein may be made without departing from the spirit and principles of the invention.

I claim:

1. A method of treating stainless steels which comprises immersing the stainless steel in an aqueous solution containing about 3 to 10% by weight chromic acid and 2 to 5% by weight commercial orthophosphoric acid acid), said solution being maintained at a temperature Within the range of about to 190 F., leaving the stainless steel part in said solution for a time of about at least 10 minutes and thereafter removing the stainless steel part from said solution.

2. A method of treating stainless steels which comprises immersing the stainless steel in an aqueous solution containing 3 to 5% by weight chromic acid and 2% by weight commercial orthophosphoric acid (85 acid), said solution being maintained at a temperature within the range of 170 to 190 F., leaving the stainless steel part in said solution for a time of at least about 10 minutes and thereafter removing the stainless steel part from said solution.

3. A method of treating stainless steels which comprises immersing the stainless steel in an aqueous solution containing 3 to 5% by weight chromic acid and 2% by weight commercial orthophosphoric acid 85 acid), said solution being at a temperature of about F., leaving the stainless steel part in said solution for a time of about 20 minutes and thereafter removing the stainless steel part from said solution.

4. A method of treating free machining stainless steels containing sulphur or selenium which comprises immers-' ing the stainless steel in an aqueous solution containing 3 to 10% by weight chromic acid and 2 to 5% by Weight commercial orthophosphoric acid (85% acid), said solution being maintained at a temperature within the range of 170 to F., leaving the stainless steel part in said solution for a time of at least approximately 10 minutes and thereafter removing the stainless steel part from said solution. Y

;5. A solution :Eor treating stainless steel to improve the corrosion resistance thereof which consists essentially of an aqueous solution containing 3 to 5% by weight chromic acid and 2% by weight orthophosphoric acid (85% acid). p

6. A method of treating stainless steel to improve the corrosion resistance thereof which comprises immersing the stainless steel for a short period of time in an aqueous solution containing 3 to by weight chromic acid and 2 to 5% by Weight orthophosphoric acid (85% acid), said solution being at a temperature within the approximate range of 170 to 190 F.

7. A solution for treating free machining grades of stainless steel to improve the corrosion resistance thereof which consists essentially of an aqueous solution containing 3 to 5% by weight chromic acid and 2% by weight orthophosphoric acid (85% acid), said solution being at a temperature of about 180 F, I

8. A method of treating free-machining stainless steel containing sulphur or selenium which comprises immersing the stainless steel in an aqueous solution containing 3 to 5% by weight chromic acid and 2% by weight comrnerical orthophosphoric acid acid), said solution being maintained at a temperature within the range of to .190 F., leaving the stainless steel part in said solution for a time of at leastapproxirnately ten minutes and thereafter removing the stainless steel part from said solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,163,583 Buzzard June 27, 1939 2,303,242 Tanner et a1. Nov. 24, 1942 2,446,060 Prey et al July 27, 1948 2,563,430 Spruance Aug. 7, 1951 FOREIGN PATENTS 398,180 Great Britain Sept, 4, 1933 

1. A METHOD OF TREATING STAINLESS STEELS WHICH COMPRISES IMMERSING THE STAINLESS STEEL IN AN AQUEOUS SOLUTION CONTAINING ABOUT 3 TO 10% BY WEIGHT CHROMIC ACID AND 2 TO 5% BY WEIGHT COMMERCIAL ORTHOPHOSPHORIC ACID (85% ACID). SAID SOLUTION BEING MAINTAINED AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 170 TO 190*F., LEAVING THE STAINLESS STEEL PART IN SAID SOLUTION FOR A TIME OF ABOUT AT LEAST 10 MINUTES AND THEREAFTER REMOVING THE STAINLESS STEEL PART FROM SAID SOLUTION. 